If this was the case a two body system should always have a circular orbit.
There are a number of highly-eccentric orbits that aren't anywhere close to circular, but they're more difficult to maintain over time, as they're more likely to be biased by other objects.
For some spacecraft (eg, STEREO; watch the first movie), they actually use this behaviour so that the spacecraft orbit the earth such that the moon will effectively 'throw' the spacecraft where they're trying to get it.
The Voyager spacecraft used this multi-planet Gravitational Assist known as Planetary Grand Tour to achieve solar escape velocity with very modest fuel requirements.
It is not uncommon to use such assists as it can radically reduce the fuel costs of a mission (to the point of making it even possible), in exchange for increased mission durations.